In a general manner, the present invention concerns operations for sealing and cutting at least two superimposed flexible sheets.
In this field, documents EP 333390 and GB-A-952581 disclose ultrasonic sealing devices between an anvil and a sonotrode oscillating at ultrasonic frequency, of at least two superimposed sheets made out of plastic material, which move continuously along a specific line.
The ultrasonic sealing devices described in these aforesaid documents have, as a common feature, an interstice for passing through two superimposed sheets to be sealed, formed between the opposite surfaces of the sonotrode and the anvil, which has a section that decreases and then increases between its input and outlet.
According to EP 333390, this configuration for the section of the passage interstice is exclusively done so that the force applied by the device to the surfaces to be sealed is homogenous along the whole length of the sealing interstice.
But nothing indicates that the configuration in EP 333390 allows the width of the seal lines to be reduced in an appreciable manner.
Moreover, the device in EP 333390 is not intended for cutting the sheets to be sealed. Instead, it is made clear that for applications of the device described in this document any cutting or perforation of the materials to be sealed is to be avoided.
Nevertheless, it has already been mentioned that it would be desirable to combine a cutting operation with the sealing of the sheets, in particular in the widespread case where one wishes to carry out, at the same time:
A cut to make the sheets of the pouch from the films that are fed through in a continuous manner.
And the sealing of the edges of the folded sheet, or the superimposed sheets, in order to make the pouch.
Thus, the device in EP 333390 does not allow the objectives of manufacturers as described above to be met in a satisfactory manner (significant reduction in the width of the seal, while at the same time allowing a high production rate and ensuring very good mechanical strength and leak tightness, as well as making it possible for a cutting operation to be carried out simultaneously with the sealing).
The same holds for the device in GB-A-952581.
In fact, quite to the contrary, as regards the width of the seal line, it is pointed out that the device described in this document is intended to maintain the seal line relatively large, by increasing the width of the contact zone between the sheets to be sealed and the device (the width of the seal line is, in the case of the device in GB-A-952581 always less than the width of the contact zone between the sheets and the device).
Furthermore, this document does not mention the possibility of carrying out a cutting operation on the sheets to be sealed.
Furthermore, EP 498 364 discloses a sealing/cutting device for sheets fed through in a continuous manner.
In this device, a cutting blade is placed downstream of the sealing sonotrode along the sheet feed direction. The sonotrode and the blade constitute two separate elements that are used, respectively, for sealing and cutting operations.
A disadvantage linked to this type of configuration is that it is very tricky obtaining a thin seal line: the cutting blade positioned downstream of the sonotrode has in fact to act on the middle of the seal line.
The seal line therefore has to be wide enough to ensure that the cutting blade does indeed act on the middle region of said line.
Besides, it is pointed out in EP 498 364 that the reduction in the width of the seal lines is not at all an objective intended for the device in this document; on the contrary, xe2x80x9cseal lines that are wider than those obtained by conventional methodsxe2x80x9d are sought (c.f. col. 4 1. 52-54 of this document).
Furthermore, a further disadvantage of the device in EP 498 364 is that it is tricky to obtain very good symmetry in the cutting profile of the sheet; in other words, obtaining the same width of cut along both sides of the seal line. This disadvantage also derives from the fact that the seal and the cut are carried out in a sequential manner, and not simultaneously. This can lead to production irregularities.
Finally, documents exist that disclose ultrasonic devices to carry out not only sealing but also cutting of films. An example of this type of device may be found, for example, in FR 2 665 683.
However, the device described in this document does not allow films fed through in a continuous manner to be processed, and so the throughput rate is limited by the discontinuous character of the process. Furthermore, no mention is made in this document of the width of the seal lines.
More specifically, according to a first aspect, the invention concerns a type of device that comprises a sonotrode and an anvil arranged opposite each other so that their facing surfaces form an interstice for passing through the sheets to be sealed, with an inlet section and an outlet section, the sonotrode vibrating at high frequencies to emit ultrasonic waves.
The invention also concerns, according to a second aspect, a sealing and/or cutting method employing the aforesaid device, where said method may be used to seal and if appropriate cut two flexible films or sheets.
More specifically, the advantageous applications of the method according to the invention concern:
sealing and if appropriate also cutting the two lateral edges of flexible packages using the aforesaid type of device;
sealing a packaging as described above using the aforesaid type of device.
It also concerns a flexible package produced specifically according to the aforesaid methods.
Such a package may be made from single-layer or multi-layer plastic films.
An advantageous but not limitative application of the invention is the manufacture of flexible pouches or sachets containing foodstuffs, for example animal foodstuffs, where said pouches or sachets are generally intended to be hermetically sealed and heat treated after being filled and sealed (in the text that follows, both the terms xe2x80x9cpackagexe2x80x9d and xe2x80x9cpouchxe2x80x9d will be used indiscriminately to designate flexible packages).
Once filled, these pouches may be sterilized by heating under pressure in a humid atmosphere.
In order to be able to withstand this type of heat treatment, they are made from one or several film(s) with a specific structure.
More specifically, they are generally manufactured from one or several film(s), each film comprising an upper layer and lower layer intended to form the external and internal faces of said packages made out of plastic material, and a central layer, sandwiched between said upper and lower layers, made out of light metal.
Even more precisely, the films from which these packages are manufactured may have the following characteristics:
the upper layer is generally a layer of polyethylene terephthalate, combined or not with a layer of polyamide;
the central layer comprises a gas tight barrier material, in particular aluminium, and the internal layer is a layer of polypropylene combined or not with a layer of polyamide;
between each layer, a spread coat adhesive is provided that allows the different layers to be bonded together;
moreover, the layer of polyethylene terephthalate may be printed on its internal face in order to decorate the exterior of said pouch. In fact, the layer of polyethylene terephthalate is generally used as a printing support and determines the exterior appearance of the pouch;
the central layer of aluminium forms a gas tight barrier, particularly to oxygen and water vapor, in order to isolate from the exterior the materials contained within the pouch;
the polyamide layer constitutes an anti-perforation layer when this is required, depending on the subsequent applications of the pouch, and the polypropylene layer acts as a scaling agent for making the pouch, and provides the general mechanical strength of the pouch and ensures leak tightness at the level of the seals of said pouch.
Such pouches may contain solids, moist solids, liquids or even a combination of solids and liquids.
At present, such pouches have various shapes.
They may be flat, with three or four seals, or with gussets with three sides scaled and one bottom sealed in the form of a gusset.
Such pouches may also have two lateral gussets, or two gussets located at the top and bottom of the pouch with sealed lateral edges.
A widely used configuration for these pouches, due to its considerable simplicity, is the xe2x80x9cflatxe2x80x9d configuration in which the pouch is simply made by assembling two sheets. This type of assembly is done in two stages:
Firstly, a pouch is made leaving one side open by sealing the two sheets on three of the four sides of the pouch (which usually has a generally rectangular shape).
To make this seal, one uses a sheet folded back on itself along a line that will constitute one edge of the pouch, or even two separate sheets that are superimposed.
In both cases, the sealing of the edges of the sheets may be combined with a cutting operation in order to separate each sheet from a plastic film.
Secondly, the last side of the pouch is closed off once it has been filled: this closing off is also achieved by sealing and may be combined with a cutting operation.
In all cases, the seals must be made in such a way as to ensure:
that the interior of the pouch has very good leak tightness not only in respect to liquids but also gases (particularly if the pouch contains liquid, moist or perishable ingredients);
that the pouch has very good mechanical strength.
The sealing operations used to make these pouches generally involve conduction sealing technology where the edges of the sides of each pouch are brought together in order to heat the internal polypropylene layer of each side by conduction through the central aluminium layer.
The conduction sealing is followed by a cooling operation, which strengthens the seal made in this way.
The seal line obtained using this type of process is generally several millimeters wide, in order to ensure the seal is both leak tight and resistant.
However, it would be advantageous to be able to reduce the width of the seal line in order to be able to reduce the amounts of material used.
In fact, the pouches are generally manufactured in very large numbers and so it is of interest to manufacturers to look for any savings that may be made on the unit cost price of the pouches; the relatively high width of the seal line thus constitutes a first disadvantage of conduction sealing.
Moreover, the implementation of this conduction sealing process with a continuous throughput of plastic films is problematical, especially due to the friction existing between the non-moving and moving parts of the sealing device.
This type of process is therefore generally used in a discontinuous mode, on films moving in a step by step manner.
Furthermore, the principle of step by step sealing allows a limited production rate. Even if two pouches are moved forward simultaneously in each step, this process reaches its mechanical working limits, notably due to the fact that the pouches have to be slowed down and then speeded up before and after the sealing unit, and the system has a certain inertia.
In order to increase the sealing rate of such packages or pouches and, for example, go from a rate of 120 packages/minute (present limiting rate for heat sealing processes) to a much higher rate (of around 500 packages/minute if not more), it has been suggested using a ultrasonic sealing device with a continuous throughput of packages.